There has so far been put to practical use a hydraulic apparatus as disclosed in Japanese Laid-Open Patent Application No. SHO 56-80554 wherein a variable displacement pump driven by an engine is controlled in accordance with the equi-horsepower characteristics A as shown in FIG. 4, and the flow Q of the fluid discharged by the pump is cut off in accordance with the cut-off characteristics B as shown in the same drawing, when the delivery pressure P of the pump reaches or exceeds a preset pressure P.sub.A.
When the above-mentioned pump is controlled in accordance with the above-mentioned equi-horsepower characteristics A, a fixed horsepower which is input to the pump can be drived therefrom as a force at a time (when P is high and Q is low), or as a speed at a time (when P is low and Q is high).
Further, by cutting off the flow Q of the fluid discharged by the pump, the fluid flow from the pump at the time of relief can be reduced so that the relief losses can be reduced.
Meanwhile, in case of hydraulic apparatuses for construction machines, the above-mentioned hydraulic pump is connected to a plurality of implement driving hydraulic actuators, such as, for example, hydraulic piston cylinder units and hydraulic motors, etc. In this hydraulic apparatus, if one implement conducts a heavy load operation so that the delivery pressure P of the pump reaches or exceeds a preset cut off-pressure P.sub.A, then the flow of the fluid discharged by the pump is reduced due to the cut-off so that the pump becomes unable to input the horsepower defined by the equi-horsepower characteristics A.
FIG. 5 illustrates rated horsepower characteristics of an engine, i.e., horsepower characteristics available when the accelerator of the engine is at its full open position. Further, this horsepower characteristics are set by the action of a governor.
If the horsepower input by the pump which is determined by the equi-horsepower characteristics A shown in FIG. 4 is matching with the horsepower developed by the engine at a point m.sub.1 as shown in FIG. 5, then a reduction in the horsepower input by the pump due to the above-mentioned cut-off results in the point where the horsepower input by the pump matches with the horsepower developed by the engine will move to a point m.sub.2, for example.
As is apparent from FIG. 5, with the reduction in the horsepower developed by the engine from the point m.sub.1 to the point m.sub.2, the number of revolutions of the engine will also change from N.sub.1 to N.sub.2, the change in the number of revolutions of the engine bringing about the following disadvantage.
Stating in brief, because the number of revolutions of the engine will increase in spite of the fact that a heavy load operation is carried out by an implement, the operator will have a feeling of disorder as if the loading on the engine were reduced, thus lowering the operational efficiency or overloading the engine or other machines thereby involving a risk.